Two important processes that are repeated frequently during the manufacture of integrated circuits are photoresist patterning and etch transfer of the pattern into an underlying substrate. As the size of circuits shrinks in order to keep pace with demand for higher performance, the patterning and etching steps become more difficult and require a higher degree of control.
To achieve a higher resolution pattern in a photoresist, hereafter referred to as resist, the exposing wavelength has decreased according to an improvement predicted by the Raleigh equation, R=kλ/NA where R is the resolution of a feature printed in a lithographic process, k is a process constant, λ is the wavelength of radiation used to expose the resist, and NA is the numerical aperture of the exposure tool. With the progression of technology nodes that require an ever smaller critical dimension to be formed in a resist, the exposure wavelength has been continually decreasing. Typically, for technology nodes above 300 nm, i-line (365 nm) and Mid-UV (436 nm) radiation is preferably used. Deep UV exposures are preferred for the 130 nm, 180 nm, and 250 nm nodes while 193 nm radiation is the leading candidate for the 100 nm node.
Resolution enhancement techniques (RET) that effectively lower the k factor in the Raleigh equation have become popular in the industry. Off-axis illumination, phase shift masks, and optical proximity correction are a few of the widely accepted RETs. A phase shifted mask (PSM) such as the one described in U.S. Pat. No. 6,306,558 not only provides improved resolution but also enhances the depth of focus (DOF) during the patterning step. This particular PSM does not contain opaque material and therefore avoids problems commonly observed with conventional binary masks where a hole in an opaque region is printed as a defect in a resist layer. Further improvement is achieved by exposing through one PSM which has a 90° shift in one half of each contact hole and a 270° shift in the other half of each hole and then exposing through a second PSM with a similar feature except that the first half of a hole region is rotated 90° relative to the first half of a hole region in the first PSM.
Another method of improving process window during a lithography process is described in U.S. Pat. No. 6,337,175. A positive or negative resist is DUV patterned to give 180 nm L/S features. A water soluble polymer containing an acid generator and optionally a crosslinker is coated on the developed image. Then a second mask with a coarser pattern is used to selectively remove some of the resist lines. The method especially improves the DOF for forming isolated lines and also suppresses a proximity effect that occurs with conventional patterning where isolated or semi-isolated lines are printed at different sizes than dense lines even though the line sizes on the mask are the same.
Unfortunately, as higher resolution resists have been developed for shorter exposing wavelengths, the etch resistance of the polymer component has decreased and the capability of the resist to serve as an etch mask for a subsequent etch step has diminished. This trend is an even larger concern since the resist is usually coated at a thinner thickness as the λ decreases in order to maintain an adequate process window during the pattern forming step. Generally, the resist thickness is no more than about 3 or 4 times the critical dimension or smallest feature size that is printed in the pattern. When the aspect ratio (feature height/feature width) becomes larger than about 3 or 4, then there is a tendency for line features to collapse during the develop and DI water rinse stage of pattern formation.
Usually, a high etch resistance has been associated with aromatic content in the polymer. Phenol groups that provide good dissolution character for Deep UV, i-line, and Mid UV resists also offer good resistance to plasma etch chemistries such as those involving fluorocarbon gases. However, for sub-200 nm exposure wavelengths like 193 nm, aromatic polymers are not useful because of a high optical absorbance. Instead, acrylate polymers and maleic anhydride/cyclic olefin (COMA) copolymers are being developed for their good lithographic properties. Although COMA copolymers are better than acrylates in terms of etch resistance, they are no match for aromatic polymers in DUV and i-line resists. It is possible that better materials can be developed for 193 nm and other sub-200 nm exposing technologies such as 157 nm, but a method is needed immediately that will allow current 193 nm resists to be incorporated into manufacturing schemes.
A large majority of Deep UV and sub-200 nm lithography processes are based on a chemically amplified (CA) resist mechanism in which one photon causes many chemical events in a resist layer. The amplified nature of a CA resist provides for higher throughput during the exposure process but is susceptible to a larger line edge roughness (LER). LER is evident during top-down views through a scanning electron microscope (SEM) inspection that is typically performed during the manufacturing process to ensure that the exposure dose and focus setting for the resist exposure are providing a quality image with the correct feature size. Frequently, jagged edges on a resist line can be seen through an SEM. This type of defect is unacceptable because the LER will be transferred into an underlying substrate and will degrade device performance. In the case of contact holes, a so-called “bird's beak” problem is evident in which pointed edges protrude from the circular opening formed in a substrate. The higher contrast property in mature CA resists that enables a high resolution feature to be printed also leads to a larger LER, especially for 193 nm exposures. Therefore, a method that can take advantage of a high resolution pattern and overcome deficiencies such as LER and bird's beak defects during an etch transfer of the pattern into a substrate is highly desirable.
FIGS. 1a-1d depict a prior art method of forming contact holes in a resist and transferring the pattern into an underlying layer. A dielectric layer 11 is formed on a substrate 10 as shown in FIG. 1a. A resist solution is coated and baked to form layer 12 and is then exposed to radiation 16 through a mask 13 containing regions 14, 15. When mask 13 is a binary type, region 14 represents an opaque coating on a transparent substrate while region 15 is transparent substrate. If mask 13 is a phase shifting type, then region 14 transmits light that is 180° out of phase with light transmitted through region 15. After exposed resist 12 is developed in aqueous base, contact holes 17a, 17b are generated as shown in FIG. 1b. This resist pattern serves as an etch mask for transferring the hole openings 17a, 17b into the dielectric layer 11 as depicted in FIG. 1c. A considerable amount of resist thickness is usually lost during this step. After the resist 12 is stripped, a top view of the resulting contact holes 17a, 17b in dielectric layer 11 as shown in FIG. 1d indicates jagged edges or bird's beaks 18 as a result of a low etch resistance of photoresist 12 and edge roughness in the resist pattern. This result is likely to occur when the pattern resist is a 193 nm sensitive composition or when the space width of the holes 17a, 17b is near the resolution limit of the lithographic process.
A method found in U.S. Pat. No. 5,950,106 provides for improved etch resistance by coating a spin-on glass (SOG) layer beneath a resist layer. The SOG is hardened at 430° C. prior to coating the resist and is comprised of SiXOY. The etch properties of the SOG enable a thinner resist layer that leads to a larger process window. The patterned resist serves as an etch mask during a fluorine based plasma etch through the SOG. Then a chlorine based plasma etches through an underlying metal layer and also removes the resist. The SOG etches 10 times faster than the resist during the initial etch while the reverse is true during the second etch step. While this method is successful when an i-line resist is employed, the LER associated with sub-200 nm resists and some DUV exposed patterns could be transferred into the substrate during the etch steps.
A method cited in U.S. Pat. No. 5,376,227 avoids etch issues associated with CA resists by patterning an inorganic layer comprised of GeXSe1−X. The method involves a photo-doping process in which Ag ions from an underlying Ag2S/AgS2Se3 layer migrate to the GeXSe1−X layer in exposed regions. The patterned top layer then functions as an etch mask for transferring the pattern through underlying layers. Additionally, better planarization is achieved by coating the inorganic resist by a RF sputtering technique.